Tag Archives: gamma-rays

In New Eyes on the Universe I give a brief mention to the High-Altitude Water Cherenkov Observatory (HAWC). When I was writing the book, HAWC had not yet started its mission. Last month HAWC published its first image. It wasn’t a particularly striking image – just the shadow of the Moon – but it was a start. Once HAWC is complete, we can expect to see some discoveries.

The Moon’s cosmic-ray shadow, as seen by HAWC. (Credit: HAWC Collaboration)

HAWC is a gamma-ray observatory. It’s the world’s largest such observatory and, even though the observatory is not yet complete, it already holds the record for detecting the highest-energy light ever seen on Earth: HAWC can detect gamma rays with energies up to 100TeV, which is many trillions of times more energetic than visible light. By capturing such high-energy photons, HAWC will enable astronomers to learn more about phenomena such as pulsars, supernovae and feeding black holes.

The HAWC Observatory in Mexico consists of an array of Cherenkov detectors – water-filled steel tanks in which photomultipliers detect radiation emitted by charged particles passing through the water. (Credit: HAWC Collaboration)

Construction of HAWC began in 2009, at an altitude of 4100 meters on the flanks of the Sierra Negra volcano near Puebla, Mexico. At the time of writing the observatory consists of 30 Cherenkov detectors: each detector is a water-filled, corrugated steel tank some 4m high and 7.3m in diameter. Inside each tank are four photomultiplier tubes that detect the cascade of particles created when high-energy gamma rays and cosmic rays crash into molecules in Earth’s atmosphere. (The photomultipliers don’t detect these particles directly. Rather, they detect the Cherenkov radiation that is emitted whenever fast-moving charged particles pass through the water more quickly than light itself can pass through the water.) By comparing signals from the different detectors, astronomers can reconstruct some of the properties of the incoming radiation that generated the particle cascade. By August of this year, about 100 of the detectors will be fully functional and HAWC will commence continuous observations of the sky. By 2014, HAWC will consist of 300 Cherenkov detectors. It will complement beautifully the existing gamma ray facilities such as MAGIC (in the Canary Islands) and HESS (in Namibia).

In his paper Weniger analyses 3.5 years’ worth of publicly available data from the wonderful Fermi Gamma-ray Space Telescope. The Large Area Telescope (LAT) on Fermi, as I’m sure you know (and if you don’t you can read about it here), is an imaging gamma-ray telescope that covers the energy range from about 30 MeV to 300 GeV. The LAT has a very wide field of view (it can see about 20% of the sky at any time) and its continuous scans mean that it covers the entire sky once every three hours. It’s an incredible instrument.

The main purpose of the LAT is to perform an all-sky survey of high-energy phenomena such as active galactic nuclei and pulsars. But it can search for signs of dark matter, too.

If dark matter particles exist then, very occasionally, one of those particles might encounter its antiparticle and the pair will mutually annihilate (just as an electron and a positron, for example, can annihilate). The result would be something we can detect: a pair of high-energy photons, for example. We don’t know what energy those photons would possess because we don’t know the mass of the dark matter particle. But every dark matter particle will have the same mass, so all photons coming from annihilation will have the same energy. And that gives astrophysicist a signal they can look for!

Within our Galaxy, the density of dark matter particles is likely to be highest at the centre. Therefore the Galactic centre is the place where dark matter annihilation is most likely to take place. Thus the Galactic centre is the most likely place to observe high-energy photons coming from dark matter annihilation. Of course, the central regions of the Galaxy will be emitting high-energy photons from a variety of processes, but those photons will have a broad spread of energies. If astrophysicists detect a sharp spike at a particular energy then that would be excellent evidence for dark matter: no other astrophysical phenomenon that we know about could generate a narrow peak in its gamma ray spectrum.

I’m sure you are ahead of me. Weniger’s analysis of Fermi data of the Galactic centre show evidence for a line at 130 GeV. The statistical significance of this result is not strong enough to make any particular claims. Weniger’s analysis rests on data points from about 50 photons, so to reach a 5 sigma level of significance would probably require several more years’ worth of data. Furthermore, there may well be systematic errors of which Weniger is unaware (since his analysis, as mentioned above, is based on publicly available data). Nevertheless, it’s perhaps another piece of the jigsaw – but one that seems to contradict another recent piece of evidence. As I said, the picture is confusing!

One interesting aside: a dark matter particle at around 130 GeV wouldn’t be too far away from the 125 GeV peak seen by the LHC and taken to be hints of the Higgs. Could it be that the Large Hadron Collider is seeing signs of dark matter rather than the Higgs? Wouldn’t that be exciting!